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Lightning occurs during a thunderstorm when random movements in the cloud cause the electric charges of the atoms in the cloud to separate. Normally the electric charges are equally spaced throughout the cloud but when lightning occurs the negative charges acculumate at the base of the cloud and the positive charges at the top.
Since like charges repel, the negative charges at the base of the cloud repel the negative charges on the ground and push them far from the surface. This leaves a net positive charge on the surface.
The negative charge in the cloud and positive charge on the ground attract each other and are pulled towards each other. However the air between the charges is a poor conductor of electricity and this insulates the cloud and ground from each other. This prevents a complete flow of charge but the negative and positive charges are able to move closer to each other. The negative charges move the furthest since they are much lighter (electrons are 1000 times lighter than ions) and the positive charges move only a little.
When the build up of charge is large enough the positive and negative charges can over come the resistance of the air and a conductive path to the ground is made. The negative charges then rush down to the ground giving the visible lightning strike.
A single lightning strike may be as much as 1 million volts, compared to 240 volts of an electrical appliances in a house.
As the electric charges build up in the cloud and on the ground
the positive charges try to get as close to the cloud as possible.
This means traveling up the tallest building inorder to try and
complete the conductive path to the cloud.
Click on this link to find out what you should do is caught in a Lightning
Storm.
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